Coated cemented carbide product

ABSTRACT

A HIGH-STRENGTH, COATED CEMENTED CARBIDE PRODUCT COMPRISING A CEMENTED CARBIDE SUBSTRATE AND A FULLY DENSE ALPHA ALUMINUM OXIDE COATING ON THE SUBSTRATE. THE COATING HAS A THICKNESS OF FROM 1-20 MICRONS AND IS FIRMLY AND ADHERENTLY BONDED TO THE CEMENTED CARBIDE SUBSTRATE THROUGH A THIN INTERMEDIATE NONMETALLIC LAYER OF AN IRON GROUP METAL ALUMINATE. THE COATED PRODUCT COMBINES A WEAR RESISTANCE SUBSTANTIALLY AS HIGH AS ALUMINUM OXIDE CUTTING MATERIALS AND A TRANSVERSE RUPTURE STRENGTH OF AT LEAST 150,000 P.S.I. THE COATED PRODUCT IS PREPARED BY PASSING WATER VAPOR, HYDROGEN GAS AND AN ALUMINUM HALIDE OVER THE SUBSTRATE AT A TEMPERATURE OF FROM 900*-1250* C., THE RATIO OF WATER VAPOR TO HYDROGEN GAS BEING BETWEEN ABOUT 0.025 AND 2.0.

United States Patent 3,736,107 COATED CEMENTED CARBIDE PRODUCT Thomas E.Hale, Warren, Mich., assignor to General Electric Company No Drawing.Filed May 26, 1971, Ser. No. 147,240 Int. Cl. B22f 7/00 US. Cl.29--182.7 9 Claims ABSTRACT OF THE DISCLOSURE A high-strength, coatedcemented carbide product comprising a cemented carbide substrate and afully dense alpha aluminum oxide coating on the substrate. The coatinghas a thickness of from 1-20 microns and is firmly and adherently bondedto the cemented carbide substrate through a thin intermediatenonmetallic layer of an iron group metal aluminate. The coated productcombines a wear resistance substantially as high as aluminum oxidecutting materials and a transverse rupture strength of at least 150,000p.s.i. The coated product is prepared by passing water vapor, hydrogengas and an aluminum halide over the substrate at a temperature of from900-1250 C., the ratio of water vapor to hydrogen gas being betweenabout 0.025 and 2.0.

BACKGROUND OF THE INVENTION This invention relates to a high-strength,coated cemented carbide product and to a process for its preparation.

Cemented carbides are Well known for their unique combination ofhardness, strength and wear resistance and are accordingly extensivelyused for such industrial applications as cutting tools, drawing dies andwear parts. It is known that the wear resistance of cemented carbidesmay be enhanced by the application of a thin coating of a highlywear-resistant material, such as, for example, titanium carbide, andsuch coated cemented carbides are finding increasing commercial utilityfor certain cutting tool and machining applications. However, theincreased wear resistance of such coated products has been at thesacrifice of the strength of the substrate which is substantiallyreduced after coating.

Because of its high hardness, wear resistance and low reactivity with awide variety of metals, aluminum oxide has excellent potential as a toolmaterial, and this potential has to some extent been realized with avariety of aluminum oxide cutting materials that are commerciallyavailable. The principal drawback to the more widespread use of aluminumoxide tools in their low strength which rarely exceeds 100,000 p.s.i.,using the standard transverse rupture or bend test. This compareswith astrength of from 200,000 to 300,000, or even more, for cemented carbidecutting tools. The low strength of aluminum oxide tools limits their useto cutting applications where the tool is not highly stressed, such asin finishing cuts. The low strength of aluminum oxide also precludes theuse of such materials in certain types of insert shapes which encounterhigh stresses When locked in a toolholder.

It is an object of this invention to provide a hard, wearresistantmaterial which combines the extremely high wear resistance of aluminumoxide with the relatively high strength and hardness of cementedcarbide.

It is an additional object of this invention to improve the wearresistance of cemented carbides without substantially reducing theirstrength. It is still an additional object of this invention to providea process for producing a firmly adherent, nonporous, dense coating ofaluminum oxide on a cemented carbide substrate.

SUMMARY OF THE INVENTION The foregoing and other objects of thisinvention are achieved by the vapor deposition under carefully con:

"ice

trolled conditions of an alpha aluminum oxide coating of from 1-20microns thickness on a cemented carbide substrate. The product containsa cemented carbide substrate and a fully dense alpha aluminum oxidecoating firmly and adherently bonded to the substrate. In addition,there is present a very thin, intermediate nonmetallic layer of cobalt-,iron-, or nickel aluminate, which acts to metallurgically bond thecoating to the substrate. The coated product has a wear resistancesubstantially equivalent to aluminum oxide base cutting materials and atransverse rupture strength of at least 150,000, in most cases greaterthan 200,000 pounds/ sq. inch. At very high cutting speeds, greater thanabout 1,500 surface ft./minute in some applications, possibly higher inothers, the higher heat resistance of solid aluminum oxide may result inhigher wear resistance. But in all cutting tests other than those abovethese levels, the wear resistance of the present coated products hasproven to be substantially as high as aluminum oxide cutting materials.

While the broad range of coating thicknesses useful in the invention isfrom l-20 microns, most coating thicknesses are preferably less than 15microns. As will be shown in more detail below, certain applicationsrequire even narrower ranges within these limits, e.g. 1-3 microns hasproven optimum for machining high temperature alloys and for millingapplications; 6-12 microns has proven optimum for steel machining.

The process of .the invention comprises passing an aluminum halide,water vapor and hydrogen gas over the carbide substrate at a temperatureof from 900-1250 C., the ratio of water vapor to the hydrogen gas beingmaintained between about 0.025 and 2.0, and preferably between 0.05 and0.20.

There have previously been references in the literature of attempts orsuggestions to coat a variety of substrates with aluminum oxide.However, insofar as is known, the coating of a cemented carbidesubstrate with aluminum oxide to produce a fully dense and adherentcoating has never previously been disclosed. Nor has the unusualcombination of properties exhibited by the present products beenpreviously attainable in either coated or uncoated cutting toolmaterials. The products of the invention are remarkable in severalrespects. Their strength as compared with comparable known coatedcemented carbide materials is considerably higher and their cuttingperformance is superior in terms of tool life at intermediate and highercutting speeds. The basis for the foregoing statements will becomeapparent from the discussion and test results set forth below.

The term cemented carbide as used herein means one or more transitionalcarbides of a metal of Groups IV b,

- Vb, and VIb of the Periodic Table cemented or bonded by one or morematrix metals selected from the group iron, nickel and cobalt. A typicalcemented carbide contain-s WC in a cobalt matrix or TiC in a nickelmatrix.

Because of the demanding requirements normally placed upon a cementedcarbide cutting material,- the properties of any coating, the manner inwhich it is bonded to the substrate and its effect on substrate strengthare extremely critical. The coating layer must have high integrity interms of density and smoothness-porosity or nonuniformity cannot betolerated. The coating must also be firmly and adherently bonded to thecemented carbide substrate to prevent spalling or separation in use. Inaddition, the coating must not reduce the strength of the cementedcarbide substrate significantly. The products of the present inventionhave been extensively tested and have been found to satisfy all of theforegoing requirements. The coatings are uniform and fully dense, theyare firmly bonded to the substrate and the coated composite retains ahigh proportion of itsstrength, usually greater than of the transverserupture strength of the uncoated substrate. The achievement of thesecharacteristics in the coated product is believed to be quiteunexpected, particularly in view of the substantial strength reductionsknown to result from the addition of wear-resistant coatings to cementedcarbide substrates. The coated materials of the invention also produce asurface finish in machining operations which appears to be fullyequivalent in quality to solid aluminum oxide cutting materials, thelatter being known to produce the best surface finishes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The outstanding properties ofthe aluminum oxidecoated product of the invention depend upon carefulcontrol of the process parameters. The process involves the use of agaseous mixture of hydrogen, water, and an aluminum halide such asaluminum trichloride. Carbon monoxide and carbon dioxide may beoptionally added. The primary overall deposition reaction is:

The most important ingredients in the gaseous reaction mixture aretherefore water vapor and aluminum chloride vapor. However, the aluminumchloride vapor can be formed in several ways during the depositionreaction, as for example by heating solid AlCl powder or by passingchlorine gas over aluminum metal. The water vapor is most convenientlyformed by reacting hydrogen with carbon dioxide in the depositionchamber to form carbon monoxide and water vapor by the water gasreaction:

H +CO =CO+H O The amount of water vapor formed in this manner dependsupon the temperature and the initial concentrations of hydrogen, carbondioxide, carbon monoxide and water vapor in the input gas stream. Inorder to form a good quality coating of desirable thickness in thetemperature range of 900l250 C., the ratio of water to hydrogen gasespresent, after the water gas reaction, should be between about 0.025 and2.0.

Hydrogen has been found to be necessary in the vapor where a=1-K; K=theequilibrium constant for the water gas reaction; )1( 2 )1+ 2)1+( 2)1+ 2)1) 2)1( 2)l+( 2)i( 2 )i 2 )l( 2)i+( 2 )l The parentheses denote theconcentration of the gaseous species enclosed within in terms of partialpressure, and the subscripts f and 1' denote the final or equilibriumconcentrations and the initial or input concentrations, respec tively.The amount of H present, and thus the H O/H ratio, can then bedetermined from the relationship:

A series of coated products were prepared in accordance with theinvention by passing aluminum chloride vapor, hydrogen and carbondioxide over cemented carbide inserts. The examples were prepared atvarious input gas compositions and at various final Hzo/Hgconcentrations. In all cases, deposition was at 1050" C. and a minutedeposition cycle was used with 2-3 grams of aluminum chloride and analuminum chloride generator temperature of about 200 C. The use of moreAlCl shifts the desired H O/H ratio to a higher value and vice versa.The coatings were deposited on a cemented carbide substrate having thefollowing composition in percent by weight: WG-72, Co-8.5, TiC-S,TaC-11.5. Table I be low shows the effect of gas composition on coatingthickness. When coating with both higher and lower ratios of H O/H (i.e.outside range of about 0.025 to 2.0), it wasnt possible to get a coatingof suflicient thickness, i.e., 1 micron. Coating quality was good forall examples having more than 1 micron thickness coating. Coatingquality was judged to be good if the coating could withstand anadherency test consisting of sliding the coated insert under a diamondbrale indentor of the same type used for the Rockwell hardness testusing a load of 2 kilograms on the diamond. If the coating resistedspalling or crumbling during this test, it was judged to have goodquality. If it did not, it was judged to have poor quality.

TABLE I Water gas reaction equilibrium Input gas partial pressurespartial pressures (Hz0)+ t1golatlng c ness z) (C 2) (C (H2O) (H2)+(H2O)+ (H2)+ (microns deposition process to obtain a dense, adherentcoating. Hydrogen appears to insure oxidation of the aluminum at thecarbide surface. Oxidation in the reaction zone above the carbidesubstrate creates a condition known as dustingwhich must be avoided. Theabsence of hydrogen creates a porous coating which is not fully dense.Thus the three necessary ingredients of the process are aluminum halidevapor, water vapor and hydrogen. In its preferred form, the processincludes the use of aluminum chloride vapor, hydrogen and carbondioxide, the latter reacting with H to form water vapor.

The amount of water vapor present, after the reaction of known inputconcentrations of H and CO and CO and H 0 if used, can be calculatedusing the following equation:

The nature of the coating obtained was determined by using X-raydifiraction analyses and optical microscopy. X-ray analyses showed thecoating to be alpha A1 0 At the higher deposition temperatures (greaterthan 1150 C.), significant amounts of the compound W CO C began to formdue to reaction of the substrate carbide with the coating atmosphere.Optical microscopy revealed a gray, translucent coating of A1 0 that wasfully dense and well bonded to the substrate in those examples in whichthe coating quality was found to be good. A very thin (less than 1micron) layer of another nonmetallic compound, cob-alt aluminate (CoAl Owas present between the A1 0 layer and the cemented carbide substrate.The presence of this thin layer is necessary to achieve the prop er bondstrength between the coating and the substrate, that is, a bond strengthsufiicient to pass the adherency test set forth above. In those cases inwhich no observable intermediate nonmetallic layer was present, thecoated inserts did not pass the above described adherency test.

For this reason, a cobalt (iron, or nickel) aluminate intermediate layeris believed necessary to a good quality coating.

The preferred temperature range for deposition of the coating is 900 C.to 1250 C. At lower temperatures,

It can be'seen that the cutting performance of the cemented carbide toolmaterial is very substantially improved by the Al O coating and thatthis improvement is substantially greater than a TiC coating on the samesubstrate. It is also evident that the amount of improvement thedeposition rate becomes very low and the coating is 5 obtained isdependent upon coating thickness up to a value poorly bonded to thesubstrate. At higher temperatures, of about 7 microns and that someevidence of performance excessive reaction occurs between the coatingatmosphere decline occurs at 10 microns. At the optimum thickness andthe cemented carbide substrate, weakening the bond value of 7 micronsfor this substrate, the performance of between the coating and thesubstrate and lowering the 10 the A1 0 coated tool was equivalent tothat of the solid strength of the overall composite body. A1 0 tool atall three speeds tested. The strength of the The strength of the A1 0coated cemented carbide A1 0 coated inserts was, however, considerablyhigher composite was measured (as were all strength measurethan solidA12 3 and higher than the Strength of the Same ments disclosed herein),using a slightly modified standard substrate with a TiC coating.transverse rupture test (ASTM No. B4066-63T), that in- 15 It should benoted that, because of strength limitations, cluded three roll loadingand a span-to-thickness ratio of it has not been feasible to use solidaluminum oxide cut- 3.5 to 1. Using a deposition temperature of 1050 C.ting materials in disposable cutting inserts of the type used and acemented carbide substrate of the nature set forth in pin-type holders.These inserts have a centrally disin the first ten examples in Table Iabove, the average posed hole for the reception of a pin which locks theinstrength of bars having coating thicknesses of from 5-7 56ft ill p TheStrength of Such inserts must e ufmicrons (the preferred thickness forthis substrate in terms ficient to resist the locking stresses. Thestrength of the f r e i t wa 241,000 p.s.i, Thi represents only presentcoated materials is sufiiciently high to enable their a slight reduction(11%) from the 270,000 strength value use in such inserts. The presentinvention therefore makes obtained from the uncoated cemented carbidesubstrate. possible the use of an insert, in such applications, havingIn the following Table II, the metal cutting performa higher wearresistance than any comparable insert presance of coated insertsprepared in accordance with this ently available. invention is shown andcompared with the corresponding The following Table III shows theperformance of the performance of uncoated inserts. Examples 11 through17 Coated inserts of the invention in cutting a high temperawere X Xdisposable cutting inserts, coated ture nickel-base alloy, specificallyInconel 718 in the soluwith A1 0 at 1050 C. by the vapor depositiontechnique tion-aged condition (B'HN 390 hardness). The results,disclosed above for Examples 1 through 10. A range of EXample 25, arecompared With the Performance f a coating thicknesses of from 1-10microns was used. These uncoated cemented carbide of the samecomposition (Exinserts were then used to machine SAE 1045 steel, 190ample 26), and in addition with a commercial solid BHN hardness, at700-1000- and ISOO-surface-feet-peraluminum oxide tool (Example 27). Theinserts were of minute speeds, .010 inch per revolution feed, and .100the negative-rake disposable type (indexable and invertiinch depth ofcut. The cutting times to a flank wear of ble) and were /2" x /2." x Thecemented carbide .010 inch are shown in Table II, along with the craterwear substrate for Examples 25 and 26 was 94% WC and 6% depth at the.010 flank wear time. The transverse rupture Co. The substrate wascoated with A1 0 at 1050 C. by strengths are also given. For comparisonpurposes, the vapor deposition process described above in connectioncutting performance and strengths of the uncoated sub with Examples 1through 10.

TABLE III Coating Time to thickness .020" flank Example Insert type(microns) Wear (min) Comments 25 A1203 coating on cemented carbide. 2. 58. 5 26.-. Uncoated cemented carbide 5. 4 27 Solid A1203- 1 Rapid edgebreakdown.

strate material, Examples 18 and 19, a commercially available solidaluminum oxide base (89% Al O 11% TiO) insert-Examples 20-22and a TiCcoated cemented car- The performance of the insert coated with 2.5microns of Al O was significantly better than that of the uncoatedcemented carbide insert of the same substrate bide insert-all run underthe same conditions-is also composition. From tests with other coatingthicknesses,

shown in Table II.

it has been determined that the optimum thickness for TABLE II QuotinCutting Time to Crater depth Transverse thickness speed, .010 flank at.010 rupture strength Example (microns) s.i.p.m. wear (min.) flank wear(p.s.i.)

11 A coating on cemented carbide 1 700 9 003" 260, 000 d0. 4 700 32 .002250,000 7 700 51 001 235, 000 10 700 51. 008" 210, 000 7 1,500 i 4. 20003" 235,000 7 1, 000 17 007" 175, 000 12 1, 000 26 003" 160,000 700 4004" 270, 000 .60.. 1, 000 5 010" 230, 000 Solid Alto 700 51 .001 90,0002l do 1, 000 30 002" 90, 000 22 1, 500 i 4. 5 0002" 90, 000 23... TiCcoating on cemented carbide 5 700 18 011" 175, 000 2A -do. 5 1, 000 4011 175, 000

1 72% W0, 8% TiC, 11.5% TaC, 8.5% Co. 9 To .004" wear.

a At .004" flank wear.

4 71% W0, 12.5% T10, 12% 'IaC, 4.5% Go.

this kind of machining (i.e., high temperature alloys) is in the 1-3micron range. Thicknesses greater than 3 microns in these testsdecreased tool life. The superior strength of the A1 coated tools isamply demonstrated by the rapid falure of the solid A1 0 tool in Example27, whereas no breakage or chipping was observed in the A1 0 coatedtools, Examples 25.

The foregoing is a description of illustrative embodiments of theinvention, and it is applicants intention in the appended claims tocover all forms which fall within the scope of the invention.

I claim: 1. A high-strength, high-wear-resistance coated cementedcarbide product comprising a. cemented carbide substrate and a fullydense alpha aluminum oxide coating of from 1-20 microns thickness firmlyand adherently bonded to said cemented carbide substrate through anintermediate layer of an iron group metal aluminate,

said coated cemented carbide product having a wear resistancesubstantially equivalent to aluminum oxide base cutting materials andhaving a transverse rupture strength of at least 150,000 p.s.i.

2. The coated cemented carbide product of claim 1 in which the cementedcarbide substrate comprises tungsten carbide and a cobalt matrix.

3. The coated cemented carbide product of claim 2 in which theintermediate layer is cobalt aluminate.

4. The coated cemented carbide product of claim 1 in the form of adisposable cutting insert for the machining of metal and othermaterials.

5. The coated cemented carbide insert of claim 4 having a centrallydisposed hole therein, the insert adapted to be mounted in a pin-typetoolholder.

6. The coated cemented carbide product of claim 1 in which the coatingis less than 15 microns in thickness.

7. The coated cemented carbide product of claim 1 in which the cementedcarbide substrate comprises titanium carbide and a matrix selected fromthe group consisting of iron, nickel and cobalt.

8. The coated cemented carbide product of claim 7 containing tantalumcarbide.

9. The coated cemented carbide product of claim 1 in which the cementedcarbide substrate comprises tungsten carbide, titanium carbide andtantalum carbide and a cobalt matrix.

References Cited UNITED STATES PATENTS 3,565,643 2/1971 Bergna 106433,110,571 11/1963 Alexander et a1. 10665 FOREIGN PATENTS 819,086 8/1959Great Britain 29182.2

CARL D. QUARFORTH, Primary Examiner US. Cl. X.R.

